examples: Add complete Go extension example with documentation (#15)

* examples: Add complete Go extension example with documentation

Adds a comprehensive Go extension example demonstrating bd's extension
patterns and Go API usage:

**New bd-example-extension-go package:**
- Complete working example in 116 lines total
- main.go (93 lines): Full workflow with embedded schema
- schema.sql (23 lines): Extension tables with foreign keys
- Comprehensive README.md (241 lines): Documentation and usage guide
- Go module with proper dependencies

**Key patterns demonstrated:**
- Schema extension with namespaced tables (example_executions, example_checkpoints)
- Foreign key integration with bd's issues table
- Dual-layer access using bd's Go API + direct SQL queries
- Complex joined queries across bd and extension tables
- Execution tracking with agent assignment and checkpointing

**Features:**
- Auto-discovery of bd database path
- Proper SQLite configuration (WAL mode, busy timeout)
- Real-world orchestration patterns
- Installation and usage instructions
- Integration examples with bd's ready work queue

This provides a complete reference implementation for developers
building bd extensions, complementing the Go API added in recent commits.

* Update go.mod after merge with main, add .gitignore

- Fix Go version to 1.21 (matches main module)
- Reorganize dependencies properly
- Keep replace directive for local development
- Add .gitignore for built binary

---------

Co-authored-by: Steve Yegge <steve.yegge@gmail.com>

examples/bd-example-extension-go/go.mod

@@ -0,0 +1,11 @@
+module bd-example-extension-go
+
+go 1.21
+
+require (
+	github.com/mattn/go-sqlite3 v1.14.32
+	github.com/steveyegge/beads v0.0.0-00010101000000-000000000000
+)
+
+// For local development - remove when beads is published
+replace github.com/steveyegge/beads => ../..